This invention relates to a method and mold for molding foamed or expanded plastic articles.
Injection molding of foamed plastic articles is becoming more economical and commercially practicable as labor and other manufacturing costs of other molding methods continue to rise. Foamed articles are light-weight, have improved strength, rigidity, and other advantageous physical properties. These articles are used in many applications, such as for appliance housings, and are generally in the form of a rigid foam plastic cellular core covered by a non-porous and non-foamed skin of the same composition.
Prior art foamed plastic articles are characterized by a poor quality skin; that is, a skin exhibiting swirls, mottled or untrue color definition, as well as surface defects such as sink marks, pin holes, cracks, and fissures. These defects are produced during injection. For example, gas released from the foamed plastic shot penetrates the skin as the cavity is being filled, resulting in the formation of swirl and surface defects. Such defects are typically repaired by relatively time-consuming and costly finishing operations.
In one known attempt to overcome these problems, molds are pre-heated to allow the injected plastic material to continue to flow into the mold and complete its expansion prior to the solidification of the skin. Because of the increased temperature of the mold, the mold cycle time is substantially increased. As a result, production rates are lowered and manufacturing costs raised.
In another known technique, molds are coated with a polymer material, such as with TEFLON coating material. This coating is intended to facilitate slippage of the plastic as it is injected, so as to minimize swirls. However, results have been unsatisfactory.
In other prior approaches, a high injection speed is used to fill the mold more rapidly. In theory, with a high enough injection speed, the mold is filled before substantial foaming takes place, so that slippage and corresponding swirl is reduced. However, in practice, articles produced in this manner still are characterized by a surface swirl pattern.
A co-injection technique is another known approach. In this technique, two resins are injected, one resin forming the skin of the article, and the other forming the core. A blowing agent (gas) is added to only the core resin so that it expands to form a cellular interior structure. The skin is formed from an unfoamed resin. Although this approach results in articles having desirable surface characteristics, relatively expensive machinery is required, and there tends to be a limit on the size of parts which may be molded.
In a still further prior technique, an expanding mold is used. In this approach, a mold is prepressurized, and a full shot, a charge that fills the entire mold, of foamable material is injected into the mold. After the skin forms, the mold is vented and expanded to permit foaming of the injected material. Although this approach does eliminate some of the surface defects in molded articles, expanding molds are typically more expensive, and not all parts are easy to form in an expanding mold.
In a prior gas counter-pressure technique, a mold is pressurized and then completely filled with molten plastic. Foaming in the mold occurs in compensation for volume contractions which take place as the plastic cools. However, with this approach, it can be difficult to match the contraction and the expansion so that the entire mold is full when the article is finished.
In an additional prior process, as exemplified by U.S. Pat. No. 4,229,395 of Nagumo, et al, a molding process is disclosed which includes the steps of prepressurizing a mold, injecting a short shot (a charge that does not entirely fill the mold) of foamable material, releasing pressure from the mold to allow the material to foam, and cooling the mold, at least during the injection step. However, several deficiencies are exhibited by such a method. For example, in a mold having a complicated angular shape, it can be difficult for the material to fill the mold completely.
Therefore, a need exists for a method and a mold for forming foamable plastic articles directed toward solving these and other problems.